There are a number of pharmaceutical compositions which suffer from instability problems due to the fact that the active component is susceptible to certain types of degradation, thereby diminishing their attractiveness and, in some cases, rendering them unsuitable from a commercial standpoint. For example, several ACE (Angiotensin Converting Enzyme) inhibitor-containing compositions suffer from this drawback since certain ACE inhibitors degrade readily in pharmaceutical dosage forms. More particularly, and as is the case with other ACE inhibitors such as Quinapril and Enalapril, Spirapril degrades readily in dosage forms to the diketo piperazine (the internal cyclization product) and the diacid (the ester hydrolysis product). Accordingly, in view of their usefulness in treating hypertension, a number of research endeavors have been directed to overcoming the instability problem associated with ACE inhibitor-containing compositions, without appreciable success.